1. Field of the Invention
The present invention relates to a method of operating an electrostatic precipitator.
An electrostatic precipitator (abbreviated ESP) is a system for collecting solid particles, which operates by virtue of the movement of charges immersed in an electric field. An electrostatic precipitator has particular utility towards cleaning of flue gasses, smokes, etc. in order to remove particles of dust, ashes, soot, and the like. The gasses are made to pass through a zone wherein an electric field is directed transversely to the flow. The electric field is operated at a high voltage where a corona of free electrons is emitted from the negative electrode. The electrodes charge the particles and the charged particles will migrate under the effect of the electric field towards the positive electrode, usually designed in the form of collecting plates on which the particles deposit. On electric discharging of the particles at the positive electrodes and possibly aided by shaking the plates, the collected dust particles fall into a hopper located below the plates.
The collecting plates are usually grounded whereas the negative electrodes are constituted of thin metallic wires maintained at a high negative potential with respect to the plates.
By virtue of electrode geometry the electric field has a higher intensity adjacent the wire electrodes, which causes the ionization of the gas in the immediate surroundings and the creation of a corona. Towards the collecting plates the electric field is distributed over a larger area with a corresponding decrease of intensity. This lower intensity electric field may not be sufficient for the ionization of the gas but serves the purpose of advancing the charged particles of dust towards the collecting plates.
In a first approximation of electrical properties, the electrostatic precipitator may be represented by a capacitor with a shunt resistance that represents the leakage by the transport of charged particles between the electrodes. In order to produce ionization of the particles the electric voltage must surpass a certain minimum threshold referred to as the corona onset voltage. Upwardly the voltage will be limited by various factors depending on the mode of operation. One of these factors may be the formation of a sparkover between the electrodes, which may take the form of a short discharge or the form of a prolonged arc. Another factor recognized in the field is the formation of corona from points on the positive electrode referred to as back-corona. Back-corona represents an increase in the leak current and impairs the particle collection efficiency.
2. Description of the Prior Art
EP patent 0 286 467 suggests a power supply wherein the power fed from the mains grid into a step-up transformer is controlled through phase angle controlled thyristors, thus producing on the high voltage side pulses at double the mains frequency. The pulses charge the electrostatic precipitator to a varying voltage. According to this publication, a detection procedure is carried out at preselected time intervals wherein the power supply is blocked for a selected interval, such as from 0.1 to 5 seconds, and then resumed. The minimum values of the pulsed precipitator voltage is observed and the presence of back-corona is established if the minimum values observed after the blocked interval exceed the minimum value observed prior to the blocking interval by a detection sensitivity factor.
U.S. Pat. No. 5 311 420 suggests a power unit comprising mains powered silicon controlled rectifiers feeding into a step-up transformer. The power supply may run in intermittent energization mode wherein the precipitator is energized by a half cycle voltage pulse followed by a predetermined number of off cycles, the ratio of on to off half cycles being optimized to prevent back-corona. The back-corona condition is detected by detecting a lack of increase of the minimum peak values of output voltage of the high voltage rectifier coincident with an increase in an output current value.
U.S. Pat. No. 4 779 182 provides an inverter power supply with switches which may be operated to output a high frequency alternating current, alternating at a frequency from 1 to 3 kHz. The feed voltage may be specified and also the voltage ripple, i.e. the voltage fluctuation between an upper and a lower limit may be specified. The direct current taken from the high voltage rectifier can be interrupted by periodic blocking in order to enforce voltage ripple on the electrostatic precipitator.
EP patent 066 950 suggests a power supply effectively comprising two complete sets of thyristor controlled high voltage power units. The first set outputs a stable base voltage whereas the second set fires single pulses to be superimposed on the back ground level provided from the first set. The electrostatic precipitator voltage takes the form of a stable back ground level superimposed with pronounced spikes. The pulse duration is within the range 50 to 200 microseconds. WO-A1-9011132 discloses a method of operating an electrostatic precipitator, wherein the power fed to the high-voltage transformer primary is controlled by thyristors fed from the mains and variation of the pulse frequency is implemented by igniting the thyristors for every third, every fifth, every seventh etc. half-cycle. Thus this method only permits varying the OFF-time intervals. The precipitator voltage values measured are the voltages at the peak, at the end of the current pulse and at 1.6 ms after the end of the current pulse.
WO-A1-9310902 discloses a method where the power fed to the high-voltage transformer primary is controlled by thyristors fed from the mains. The voltage is measured 1-3 times per ms. A xe2x80x9cfigure of meritxe2x80x9d is established using a formula involving the time integral of the square of the voltage. Variation of the pulse frequency is implemented by igniting the thyristors for only part of the half-cycles and by controlling the firing angle.
In operating conditions of high resistivity dust, the dust deposited on the plate electrode will resist discharging of the ionized particles. The voltage tends to increase across the dust layer, and to correspondingly decrease across the gas. If the voltage across the dust layer continues to build up, a point is reached where a dielectric break down through the dust layer occurs. This point is known as the onset point of the back corona discharge. The dielectric break down of the dust layer produces positive ions, which decrease particle charging, and result in a reduction of the collection efficiency.
The formation of back-corona takes some time, and this is related to the relaxation time of the dust layer.
As the dust layer can be considered as a leaky capacitor, it will tend to smooth out the current pulses delivered to the electrostatic precipitator. This effect may be put to advantage as short pulses may be applied to the electrodes without prompting the formation of back-corona on the dust layer. Rather the initiation of a back-corona situation seems to be governed by the time average value (mean value) of the precipitator current.
Therefore, in order to avoid or reduce the back-corona discharges, the mean current delivered to the precipitator has to be decreased. The problem is to do this without losing too much voltage level.
The basic control problem is then to determine the current that has to be delivered to the precipitator in accordance with the existing operating conditions. For some industrial processes, the dust resistivity can sometimes be low and sometimes be high, causing back-corona. In the first case the current has to be as high as possible, and in the second case the current has to be reduced.
The traditional power supply for ESP""s used until now is a transformer rectifier set, consisting of a high voltage transformer and a bridge rectifier. A pair of antiparallel thyristors using phase angle control controls the primary voltage applied to the HV transformer.
A non-linear resistance in parallel with a capacitance may represent the ESP load. The capacitance for a medium size ESP bus-section is 60-80 nF (2000 m2 collecting plate area). This means that the time constant of the load is in the millisecond range, causing the waveform of the voltage applied to the ESP to contain a considerable ripple. Therefore the voltage applied to the ESP can be characterized by its mean value, peak value and trough (minimum) value. The ripple is expressed as the peak value minus the minimum value.
The current delivered to the ESP consists of rectified sinusoidal-alike pulses whose amplitude and duration depend on the value of the phase angle. For normal conditions (no back-corona) an increasing current gives an increasing voltage mean value and voltage ripple. The current pulses has a duration shorter than the period of the line frequency (10 ms for a 50 Hz-line), but in case of very high dust resistivity the electrical charge delivered in one current pulse may be high enough to start back-corona discharges.
Furthermore, the occurrence of sparks, arcs and short-circuits inside the ESP cause current surges in the line current, which are normally limited by the inclusion of a linear inductance in series with the primary circuit.
The problems can be avoided by using a new type of power supply known as switch mode power supply (SMPS), operating at a switching frequency above the audible limit. The current delivered by an SMPS is pulses of short duration, in the range of 10 to 30 microseconds. This solution consists basically in replacing the phase control thyristors by a rectifier and a DC-AC inverter connected between the mains and the transformer rectifier, which in this case has to be designed to cope with high frequency. Among the various types of inverters available, it has been found that a series-resonant inverter provides several advantages in relation to ESP energization.
Such an inverter with an inductance and a capacitance in series makes it possible to deliver rectified sinusoidal current pulses to the ESP with a duration of 10 to 30 microseconds and provides natural current commutation. Moreover, by choosing the values of the series inductance and capacitance, it turns out that the duration and the amplitude of the current in the main circuit of the inverter and in the primary of the HV-transformer are only determined by these components and become independent of the ESP load.
Thus, this SMPS has the advantages of being capable of delivering electrical charge to the ESP in small amounts and of avoiding current surges as the current amplitude is determined by the resonant components of the inverter and not by the ESP load. In case of a short-circuit inside the ESP, the amplitude of the primary current is unchanged, and the line current falls to a low value. This beneficial effect is due to the fact that the mains have only to deliver power to cover the losses in the power supply, as the output power is zero.
This type of power supply has also another important feature. By using one or few current oscillations and then interrupting the power for a certain time the voltage waveform can in practice be a pure DC-voltage (no AC-component).
Furthermore, by operating the inverter to generate current oscillations during a longer time interval, e.g. during 1 to 2 milliseconds, the so-called ON-time, the precipitator voltage can be raised at a higher rate of rise compared with traditional energization. Thereafter the current oscillations are interrupted during a so-called OFF-time, where the precipitator voltage falls exponentially towards the corona onset value. In other words, this type of SMPS can produce different voltage waveforms on ESP loads, ranging from a practically pure DC-voltage to a very steep and pulsating voltage.
The invention, in a first aspect, provides a method of operating an electrostatic precipitator, comprising the steps of intermittently feeding the precipitator with electric power according to a cycle comprising a controlled preset ON-time interval and a preset OFF-time interval in order to apply to the precipitator electrodes a cyclic time-varying voltage, monitoring the electrode voltage and establishing a voltage peak value and a voltage mean value, multiplying the established peak value with the established mean value to compute an index of expected performance (IEP), effecting successive incremental time variations of said ON-time interval and repeating the steps of monitoring and multiplying so as to establish a correlation of said index to said time variations, establishing a time value of said ON-time interval corresponding to a maximum of said index, and selecting said established time value as a new set point for said ON-time interval.
The invention, in a second aspect, provides a method of operating an electrostatic precipitator, comprising the steps of intermittently feeding the precipitator with electric power according to a cycle comprising a controlled preset ON-time interval and a preset OFF-time interval in order to apply to the precipitator electrodes a cyclic time-varying voltage, monitoring the electrode voltage and establishing a voltage peak value and a voltage mean value, multiplying the established peak value with the established mean value to compute an index of expected performance (IEP), effecting successive incremental time variations of said OFF-time interval and repeating the steps of monitoring and multiplying so as to establish a correlation of said index to said time variations, establishing a time value of said OFF-time interval corresponding to a maximum of said index, and selecting said established time value as a new set point for said OFF-time interval.
The invention, in a third aspect, provides a method of operating an electrostatic precipitator, comprising the steps of intermittently feeding the precipitator with electric power according to a cycle comprising a controlled preset ON-time interval and a preset OFF-time interval in order to apply to the precipitator electrodes a cyclic time-varying voltage, monitoring the electrode voltage and establishing a voltage peak value and a voltage mean value, multiplying the established peak value with the established mean value to compute an index of expected performance (IEP), effecting successive incremental time variations of said ON-time interval simultaneously with effecting successive incremental time variations of said OFF-time intervals and repeating the steps of monitoring and multiplying so as to establish a correlation of said index to said time variations, establishing a time value of said ON-time interval and a time value of said OFF-time interval corresponding to a maximum of said index, and selecting said respective established time values as new set points for said ON-time interval and said OFF-time interval.
The inventor has found that in adverse operating conditions, i.e. back-corona, and also in normal conditions, a pulsating precipitator voltage with a high rate of rise plays an important role in the collection efficiency.
The mean current can be controlled by means of the ON-time and the OFF-time, and the present invention deals with the control strategy for the determination of the appropriate values for the two time intervals, leading to the best collection efficiency for particular operating conditions of the precipitator.
The particle charging is proportional to the peak value of the precipitator voltage, while the force exerted on the charged particles for their removal from the gas stream is proportional to the mean value of the precipitator voltage. The inventor has found a good correlation between the particle collection efficiency and the product of the peak value and the time average of the precipitator voltage, so the control strategy should preferably be based on a criterion of maximizing the product of these two factors.
This is achieved by the invention as summarized above.
The method according to the invention provides an optimal strategy for selecting the best operating parameters, thereby improving collection efficiency. Further, the procedure for searching the optimum does not require departing from operating the ESP close to the optimal electrical conditions. This is advantageous in particular in view of the fact that searching in order to optimize operating parameters usually has to be carried out frequently to account for frequent variations in operating conditions. The method according to the invention permits a comparatively simple control strategy.
According to a preferred embodiment, power may be fed to the ESP intermittently, giving a pulsating voltage because of the RC nature of the ESP load. The power is delivered to the ESP as current bursts, adapted to raise the precipitator voltage at a rate of about 30 kV/ms. The substantial increase of precipitator voltage within a very short time permits the attainment of a high peak value with a comparatively lower risk of initiating a spark or a back-corona condition. On the other hand, this rate of rise is within the capabilities of a SMPS of a comparatively simple design.
The method according to the invention may be implemented using an inverter in the power supply that operates at a fixed switching frequency and with a well-defined current waveform consisting of sinusoidal pulses. This reduces the generation of higher harmonics and eliminates the current surges in the mains in case of sparks, arcs or short-circuits inside the ESP.
According to a preferred embodiment, the step of effecting successive incremental variations comprises varying the ON-time and the OFF-time, independently of each other or simultaneously.
This method is convenient in the process of finding an optimum set of operating parameters so as to ensure efficient operation. The power supply may comprise a control logic adapted to drive the solid state components so as to produce output power intermittently. This simplifies design and control of the power unit, and produces an output voltage exhibiting a low ripple content which has a favorable effect on the electrostatic precipitator efficiency.
Obviously the fact that the power supply is capable of outputting a high ripple output signal does not exclude that the power supply could be adapted with the option of switching to another function mode which might be appropriate in particular circumstances. Other function modes that are known in the art per se, may e.g. comprise a DC mode, sometimes referred to as a pure DC mode. The power supply according to the invention can easily be controlled in such way as to output a low ripple signal, e.g. by outputting a high frequency signal intermittently with a suitably fast switching between on and off phases.